In the prior art, and in particular from WO 2006/050304, a liquid injection syringe is known of the type that comprises a body of generally tubular shape forming a reservoir for the liquid and having fitted thereto an endpiece of generally annular shape for fastening a needle carrier on the syringe body.
The fastener endpiece is sometimes referred to as a “luer lock” and the needle carrier is sometimes referred to as a “luer needle”.
The syringe also includes a plunger mounted to be movable axially in the body between a ready position and an end-of-liquid-injection position.
Below, an element is said to be “proximal” or “distal” depending on whether it is axially close to or far away from the end of the plunger that is to be actuated by a user.
WO 2006/050304 also describes a safety device for a syringe of the above-specified type, the safety device being of the type comprising:                a first member of generally tubular shape, referred to below as a protective sheath; and        a second member of generally tubular shape, referred to below as a syringe support, which second member is received in the sheath, substantially coaxially therewith.        
The protective sheath and the syringe support are movable axially relative to each other between two positions, referred to below as the uncovered position of the syringe needle and the retracted position of said needle.
The safety device enables the needle to be retracted automatically into the sheath so as to ensure that a person handling the syringe assembly after it has been used normally, i.e. after the liquid has been injected into the body of a patient, cannot accidentally be pricked by the needle.
The fastener endpiece includes a proximal end that is generally engaged on a distal end of the syringe body that is generally conical in shape.
Furthermore, the fastener endpiece includes a distal end designed to co-operate with a needle carrier by screw fastening. The needle carrier and the needle it carries are generally covered prior to use of the syringe by a cap. Where appropriate, prior to putting the needle carrier into position on the syringe body, a plug for closing the syringe body is screwed onto the fastener endpiece.
The torque needed for unscrewing the plug or for screwing on the needle carrier can cause the fastener endpiece to turn about the axis of the syringe body. It is therefore necessary to be able to oppose turning of the fastener endpiece, particularly in order to be able to unscrew the plug or to screw on the needle carrier.
Unfortunately, after the syringe has been mounted in the safety device, the fastener endpiece is generally rendered practically inaccessible by the protective endpiece that covers it. The user therefore cannot act directly on the fastener endpiece to prevent it from turning, in particular while unscrewing the plug or screwing on the needle carrier.
WO 2006/050304 proposes means for preventing the fastener endpiece from turning, which means comprise in particular a pair of tongues provided in the protective sheath and in the syringe support. When the syringe and the support are in their relative uncovered position of the syringe needle, the tongue of the sheath covers the tongue of the syringe support. The tongues are elastically deformable in a radial direction. To prevent the fastener endpiece from turning, the user presses on the tongue of the sheath so as to push it radially towards the axis of the sheath, thereby pushing the tongue of the support (which is covered by the tongue of the sheath) until it comes into contact with the fastener endpiece.